In the field of optical communication, the transmission capacity per optical fiber cable has been increasing, and the wavelength division multiplex (WDM) communication has been known as one communication method for achieving the capacity increase. In the wavelength division multiplex communication, a plurality of optical signals each having different wavelengths are simultaneously transmitted through one optical fiber cable.
An optical transceiver is known as one of optical communication modules used for the wavelength division multiplex communication. The optical transceiver used for the wavelength division multiplex communication includes an optical transmitter module having at least a plurality of light emitting elements which emit lights (optical signals) each having different wavelengths and a multiplexing optical system which multiplexes the optical signals emitted from the light emitting elements. An ordinary multiplexing optical system is made up by the combination of a plurality of multi-layer filters (wavelength selective filter and reflection mirror) which transmit a light with a predetermined wavelength and reflect a light with a wavelength other than that (Japanese Patent Application Laid-Open Publication No. 2013-104983 (Patent Document 1), Japanese Patent Application Laid-Open Publication No. 2013-145356 (Patent Document 2), US Patent Application Publication No. 2012/189314 (Patent Document 3) and Japanese Patent Application Laid-Open Publication No. 2005-309370 (Patent Document 4)). The optical transmitter module is sometimes referred to as a transmitter optical sub-assembly (TOSA).
FIG. 8 shows a configuration example of a multiplexing optical system used for the conventional optical transmitter module. The multiplexing optical system shown in FIG. 8 is made up of three wavelength selective filters 101, 102 and 103 and one reflection mirror 104.
An optical signal emitted from a light emitting element 201 enters the reflection mirror 104. The optical signal which has entered the reflection mirror 104 is sequentially reflected by the reflection mirror 104, the wavelength selective filter 101, the reflection mirror 104, the wavelength selective filter 102, the reflection mirror 104 and the wavelength selective filter 103 and is then output.
An optical signal emitted from a light emitting element 202 passes through the wavelength selective filter 101 and enters the reflection mirror 104. The optical signal which has entered the reflection mirror 104 is sequentially reflected by the reflection mirror 104, the wavelength selective filter 102, the reflection mirror 104 and the wavelength selective filter 103 and is then output.
An optical signal emitted from a light emitting element 203 passes through the wavelength selective filter 102 and enters the reflection mirror 104. The optical signal which has entered the reflection mirror 104 is sequentially reflected by the reflection mirror 104 and the wavelength selective filter 103 and is then output.
An optical signal emitted from a light emitting element 204 passes through the wavelength selective filter 103 and is then output.
As described above, the optical signal emitted from the light emitting element 201 is reflected by the reflection mirror three times and reflected by the wavelength selective filters three times. The optical signal emitted from the light emitting element 202 passes through the wavelength selective filter once, is reflected by the reflection mirror twice and is reflected by the wavelength selective filters twice. The optical signal emitted from the light emitting element 203 passes through the wavelength selective filter once, is reflected by the reflection mirror once and is reflected by the wavelength selective filter once. The optical signal emitted from the light emitting element 204 passes through the wavelength selective filter once. In the following description, a total of the number of times the optical signal passes through the wavelength selective filter, the number of times the optical signal is reflected by the wavelength selective filter and the number of times the optical signal is reflected by the reflection mirror is referred to as “the number of filter transmissions”. More specifically, the number of filter transmissions of the optical signal emitted from the light emitting element 201 shown in FIG. 8 is six (six reflections). Also, the number of filter transmissions of the optical signal emitted from the light emitting element 202 is five (one transmission and four reflections), the number of filter transmissions of the optical signal emitted from the light emitting element 203 is three (one transmission and two reflections), and the number of filter transmissions of the optical signal emitted from the light emitting element 204 is one (one transmission). Therefore, the maximum number of filter transmissions in the optical transmitter module having the multiplexing optical system shown in FIG. 8 is six.